The present invention relates to the synthesis of 2-halo-4,6-dinitroresorcinol. That compound is a useful intermediate in the synthesis of monomers for making cis-polybenzoxazoles (cis-PBO).
4,6-Diaminoresorcinol and its acid salts are known to be useful monomers in the synthesis of cis-PBO. It is known to synthesize the monomer from 1,2,3-trichlorobenzene in a three-step process by:
(1) nitrating 1,2,3-trichlorobenzene with nitric acid to form 1,2,3-trichloro-4,6-dinitrobenzene; PA1 (2) contacting the product of step (1) with alkali metal hydroxide in an alkanol solution under conditions such that the chlorine atoms in the 1- and 3-positions are displaced to form 2-halo-4,6-dinitroresorcinol; and PA1 (3) contacting the product of step (2) with a hydrogen-reducing agent and a catalyst in an organic solution under conditions such that it is reduced to form 4,6-diaminoresorcinol. PA1 (1) contacting 1,2,3-trihalo-4,6-dinitrobenzene with hydroxide anions in an aqueous solution under conditions such that 2-halo-4,6-dinitroresorcinol is formed; and PA1 (2) contacting the product of step (1) with a hydrogenating agent in the presence of a noble metal hydrogenation catalyst and in the presence of an acid having a higher pK.sub.a than the hydrogen halide of the halogen in the 3-position in an aqueous solution under conditions such that 4,6-diaminoresorcinol or an acid salt thereof is formed. PA1 (1) is a weaker acid than the hydrogen halide of the halogen atom in the 2-halo-4,6-dinitroresorcinol, PA1 (2) is stable under reaction conditions, and PA1 (3) does not interfere with the functioning of the catalyst,
The process is fully described in Lysenko, High Purity Process for the Preparation of 4,6-Diamino-1,3-benzenediol, U.S. Pat. No. 4,766,244 (Aug. 23, 1988) which is incorporated herein by reference.
The known process is effective to produce 4,6-diaminoresorcinol in high yields. However, it uses large amounts of organic solvent, which is undesirable for two reasons. First, organic solvents are substantially more expensive than aqueous solvents for commercial scale production. Second, the organic solvents are flammable, which is particularly undesirable in working with the nitrated aromatic intermediates previously described. What is needed is a process to produce 4,6-diaminoresorcinol while reducing or eliminating the organic solvents used in the synthesis.